So been riding for years a 7800 crank/bb combo on my bike and always thought it was nice how snug the left crank arm was to the BB. Building another bike with a 7800 crank/bb as well, but noticed how there was a very noticable gap between the left crank arm and BB. Hmm, mis-milling? Well took the cranks off and tried both left arms on both bikes. The one fit snug and the other had the 'gap' on both bike's BBs. Examined the arms. A-ha. The one with the gap has a gasket type o-ring of sorts at the end of it thus limiting it travel as the BB has a slight taper to it. The other is missing the o'ring thing and gets nice and close. My question to the audience is: Do I need this o-ring? Thanks.